High molecular weight polyethylene terephthalate fiber forming polyesters are well-known in the art. They are prepared commercially either by the ester interchange reaction between dimethyl terephthalate and ethylene glycol, or by the direct esterification process wherein terephthalic acid is reacted directly with ethylene glycol. Products and processes of these types are described, for example, in U.S. Pat. Nos. 2,465,319, 3,050,533, 3,051,212, 3,427,287 and 3,484,410, all hereby incorporated by reference.
Fibers and cords produced from polyethylene terephthalate esters are known to exhibit excellent dimensional stability, that is, they exhibit low extension or growth during use and have resistance to thermal degradation. However, when used in applications where a high degree of heat is built up during use as in pneumatic tires and industrial belts which operate under high speed conditions under heavy load, a loss of tensile strength is often experienced because of these conditions. Efforts to remedy these problems have not always been effective.
Most research in the field has been directed to producing a high molecular weight linear polyester which has a low content of free carboxyl groups as the art is aware that the lower the content of free carboxyl groups and the higher the hydrolytic stability, the better the fibers and cords perform under high speed and heavy load conditions.
A number of U.S. patents have issued which are directed to solutions to this problem. Thus, U.S. Pat. No. 3,051,212 suggests production of a polyester having a free carboxyl content of less than 15 equivalents by treating the filaments after they have been formed with a chemical reagent which "caps" the free carboxyl group, one such agent being diazomethane.
In U.S. Pat. No. 3,627,867, the solution is to reduce the normally high carboxyls by injecting ethylene oxide or other low-boiling oxirane compound before the molten polyester is fed into the melt-spinning machine. U.S. Pat. No. 3,657,191, hereby incorporated by reference, seeks to improve stability by reacting the polyesters with ehtylene carbonates or monofunctional glycidyl ethers. Also, U.S. Pat. No. 3,869,427, hereby incorporated by reference, seeks to overcome the heat degradation problem by mixing with the molten polyester prior to melt-spinning, 1,2-epoxy-3-phenoxypropane or 1,2-epoxy-3-n-hexyloxypropane. Further, U.S. Pat. No. 4,016,142, hereby incorporated by reference, suggests reduction of the number of free carboxyl end groups also by adding a glycidyl ether to the polyester to react with carboxyl end groups which are present so as to form free hydroxyl end groups.
In U.S. Pat. No. 4,139,521, hereby incorporated by reference, there is disclosed and claimed a process for improving heat stable fiber forming linear condensation polyesters by incorporating therein a stabilizing amount of a stabilizer comprising an N-epoxyalkylimide of a dicarboxylic acid.
In the prior art, techniques have been developed to produce tire yarn having a free carboxyl concentration of less than 10 grams equivalents of carboxyl group per 10.sup.6 grams of polyester (sometimes hereinafter referred to as milliequivalents or meq of carboxyls per kg polymer).
The process of the present invention is an improvement over these prior disclosed and claimed processes in the provision of linear condensation polyesters in which there is achieved a reduction of the carboxyl content to a concentration of less than about 5 gram equivalents per 10.sup.6 grams of polyester specialty fiber.